This invention relates to a powder coating system in which an epoxy resin and a low temperature curing agent are extruded or otherwise melt-mixed as one component and pulverized to form a low temperature curable coating powder. The powder cures at a temperature of from about 225 to about 300xc2x0 F. and produces a coating having an exceptionally smooth surface with either a low or high gloss. This invention also relates to the electrostatic coating of massive metal articles and wood and low temperature curing to achieve a pleasing smoothness. It also relates to a method for compounding a coating powder by extruding a mixture comprising an epoxy resin having a low melt viscosity and a low temperature curing agent at a temperature of from about 160xc2x0 F. to about 230xc2x0 F. (about 71-110xc2x0 C.).
Traditionally, coating powders have been made by the extrusion of a mixture of resins and curing agents to obtain a homogeneous mixture and then grinding the extrudate and screening the comminuted product to obtain the desired particle sizes and particle size distribution. The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 200xc2x0 F. Achieving a powder coating composition that will cure on heat sensitive substrates at less than 300xc2x0 F. in less than 5 minutes has long been a goal of the industry. The curing of powder coatings on heat sensitive materials such as wood, plastic, and the like has been limited by the fact that the extrusion of a mixture of a resin and a low temperature curing agent, i.e., one that is active at 250xc2x0 F. or less, would cause the coating powder to gel in the extruder because the extrusion typically generates enough heat to raise the temperature to 200xc2x0 F. or higher.
This problem has been avoided by extruding the resin and a small amount of catalyst or low temperature curing agent, grinding the extrudate, and then blending the resultant powder with an additional amount of the curing agent in powder form, according to the disclosure in co-pending, commonly assigned application Ser. No. 917,043, now U.S. Pat. No. 6,077,610 filed Aug. 19, 1997.
The high temperatures that are generated by the extrusion of a curing agent with a resin are caused in part by the fact that the temperature must be sufficient to facilitate the mixing of resins having initially high melt viscosities. Even higher temperatures are produced by the friction arising from the mixing of the still highly viscous molten resins with curing agents that are solid at room temperature.
It is an object of this invention, therefore, to provide a method for extruding a one-component low temperature curable coating powder.
It is a related object of this invention to provide a one-component low temperature curable coating powder that has satisfactory blocking resistance during storage at normal temperatures.
It is a related object of this invention to provide a one-component low temperature curable coating powder for heat sensitive substrates and massive metal articles.
It is another object of this invention to provide a low temperature process for producing a smooth, high gloss coating on wood.
These and other objects of the invention which will become apparent from the following description are achieved by a method for preparing a thermosetting coating powder comprising extruding or otherwise melt-mixing an epoxy resin having a melt viscosity of from about 200 to about 3000 centipoise at 150xc2x0 C. and a curing agent which is latent at a temperature of from about 160xc2x0 F. to about 230xc2x0 F., cooling the mixture, and comminuting it.
For the purpose of describing the proportions of components in the compositions of this invention, the term resin includes the resin per se and the curing agent but not the catalyst. Also, the term xe2x80x9cone component coating powderxe2x80x9d means that the powder is fully formed by grinding and screening only one extrudate of a mixture of the resin, curing agent, catalyst, and additives.
Extrusion is but one form of melt-mixing known to the powder coating art for the thorough mixing of components that is necessary in the manufacture of the powder. It is, however, a quite convenient and efficient form of melt-mixing, i.e., mixing of the powder components while in the molten state, and is preferred in this invention. In either instance, the molten material is cooled and ground to obtain an average particle size of from about 10 to about 100 xcexc.
Epoxy resins which are suitable for the purposes of this invention have an equivalent weight of from about 100 to about 700. Mixtures of such epoxy resins may be used. A suitable mixture may comprise an epoxy resin having an equivalent weight between about 100 and 400 and one having an equivalent weight between 400 and about 700 in a weight ratio of from 1:99 to 99:1. The resins are exemplified by, but not limited to, those produced by the reaction of epichlorohydrin and a bisphenol, e.g., bisphenol A and bisphenol F. The low melt viscosities of these resins facilitate the extrusion of them in admixture with a curing agent, additives and pigments at about 160-220xc2x0 F. The preferred melt viscosity is from about 300 to about 1000 centipoise. The melt viscosity of resins having a low Tg, i.e., from about 35xc2x0 C. to about 65xc2x0 C., is suitable for the purposes of this invention. Epoxy resins known as EPN (epoxy phenol novolac) and ECN (epoxy cresol novolac) resins and those made by the reaction of isopropylidenediphenol (bisphenol A) with epichlorohydrin are suitable for the purposes of this invention. Bisphenol A epoxies are sold under the trademarks ARALDITE GT-7071, GT-7072, EPON 1001 and EPON 2042. ARALDITE GT-6259 is the trademark for an ECN resin. Epoxy resins suitable for the purposes of this invention are further exemplified by the triglycidylisocyanurate (TGIC) resins, the glycidyl methacrylate resins and glycidyl resins containing a hydrogenated bisphenol A moiety, all of which are useful when coatings having an improved UV stability are desired.
The use of a crystalline epoxy resin may improve the flow characteristics of the fused coating powder and, therefore, the smoothness of the fused and cured coating. A particularly desirable flow is achieved when a crystalline epoxy constitutes from about 5 to about 20% by weight of the total amount epoxy resin used in the formulation of the powder. The performance of a coating powder of this invention deteriorates as the level of crystalline epoxy resin therein is increased because of the relatively low equivalent weights of such resins and the preferred amount of such a resin is about 10% or less for that reason. A crystalline epoxy resin having a melting point between about 80xc2x0 C. and about 150xc2x0 C. is preferred. A crystalline epoxy resin having an equivalent weight of about 185 and sold by Shell under the trademark RSS 1407 is suitable for the purposes of this invention.
When resins having a Tg of from about 35xc2x0 C. to about 40xc2x0 C., are used in this invention, sintering of the powder is avoided by allowing the temperature in the extruder to rise to activate the low temperature curing agent for a time sufficient to raise the extrudate""s viscosity beyond the sintering point and then cooling the extrudate rapidly to about 10-20xc2x0 C. (about 50-70xc2x0 F.) before chipping and grinding it and storing the powder at such temperature to avoid a further viscosity build-up by continued curing. Another way to avoid sintering of the powder when low Tg resins are used is to pre-mix the resin with a crystalline or non-crystalline curing agent powder having an average particle size of about 5 microns that does not liquefy in the extruder. The powdered agent does not react with the resin so readily as does the same curing agent in flake form. A specific example of a curing agent that may so used in the powdered form is sold under the trademark ANCAMINE 2441.
The low temperature curing agent of this invention is one that will be active at a temperature of from about 225 to 300xc2x0 F. and may be selected from among the many that are commercially available. Polyamines in general are the active curing agents but they must be converted from their usual liquid state into a a solid that may be pulverized in order to serve in this invention. A friable solid low temperature curing agent may be selected from among the many that are commercially available but a blocked polyamine such as an epoxy adduct of an aliphatic polyamine (including cycloaliphatic polyamines) having a primary, secondary, or tertiary amino group or a combination of such amino groups is a suitable curing agent for the purposes of this invention. Suitable curing agents derived from polyamines are available from Ciba Ceigy as its HT 835 hardener and from Air Products and Chemicals under the trademark ANCAMINE 2337 XS, ANCAMINE 2014 AS, and ANCAMINE 2441 (Air Products and Chemicals). An epoxy adduct of an aromatic polyamine, such as methylene dianiline, is also a suitable curing agent for the purposes of this invention. It is preferred that the functionality of the adducting reaction mixture is 2 or less and it is particularly preferred to use a difunctional epoxy compound. The epoxy resin portion of the adduct is aromatic or aliphatic, as exemplified by the bisphenol-based resins mentioned above and the aliphatic analogs thereof, respectively. The cyclohexanol analog of the bisphenol A-based resin is available under the trademark KUKDO 4100. The polyamine is exemplified by ethylene diamine, isophorone diamine, cyclohexyldiamine, and a fluorinated diamine such as 4,4xe2x80x2-hexafluoro isopropylidene bis-aniline. Higher molecular weight polyamines are preferred when epoxy resins having a low equivalent weight (e.g., GT 7071 from Ciba) are employed. An epoxy adduct of an aliphatic polyamine having a secondary amino group, such as the ANCAMINE 2014 AS curing agent, is preferred for white and light colored coatings. Other adducts of polyamines such as a mono-amide derived from phthalic anhydride and a polyamine such as that available from Ciba-Geigy under the trademark HT-939 are also suitable as blocked polyamine friable curing agents for the purposes of this invention. A substituted urea derived from an adduct of a diepoxide and a diamine and available under the OMICURE trademark is also suitable as a friable curing agent in this invention. The amount of low temperature curing agent is from about 2 to about 40 parts per hundred parts of the resin (phr) and the preferred amount is from about 5 to about 20 phr. Increasing levels of the curing agent reduce the gel time and, therefore, increase the orange peel effect.
Protection of the polyamine moiety of the curing agent against the harmful effects of moisture is important in the operation of this invention. Encapsulation of the curing agent by spray drying a dispersion of the powder in aqueous poly(vinyl alcohol) is suitable.
A catalyst may be used at a level of from about 0.1 to about 5 parts per hundred parts of the resin, preferably about 0.2-2 phr to accelerate the curing reaction with the low temperature curing agent. Preferred catalysts for this invention are imidazoles and epoxy adducts thereof, the imidazoles having the general formula: 
wherein R1, R2, R3, and R4 are independently hydrogen, alkyl, aryl, or any substituent which is not reactive with the epoxy resin. For the purposes of this invention, the term imidazole is used herein to mean both the substituted and unsubstituted imidazoles. Imidazoles, in themselves, tend to be insoluble in epoxy resins. Thus, epoxy adducts are made to make them more compatible with the epoxy system of this invention. Suitable adducts of such imidazoles with a bisphenol A epoxy resin are available commercially from Shell Chemical Company under its trademark EPON, e.g., EPON P-101, and also from Ciba-Geigy Corporation under its designation HT 261. Examples of suitable imidazoles include imidazole, 2-methyl imidazole, and 2-phenyl imidazole. For enhanced color stability, the 2-phenyl imidazole, which is available from the SWK Chemical Co., is preferred. A particularly hard surface is obtained when a combination of the ANCAMINE 2441 curing agent and 2-phenyl imidazole is used to cure an epoxy resin having a medium Tg and melt viscosity.
Although applicants are not bound by any theory, it is believed that an imidazole adducts to an epoxy resin by an opening of the epoxy ring that results in the epoxy oxygen bonding to the Cxe2x95x90N bond of the imidazole ring. The adducted imidazole moves from one epoxy group to another as it facilitates epoxy ring openings and cure reactions.
A tertiary amine such as triethylamine diamine, available under the trademark ACTIRON SI 27071 from Synthron Chemicals, is another type of curing catalyst that may be used in this invention.
Various gloss levels for the cured coating may be achieved through the choice of epoxy resins, curing agents, curing catalysts and the relative amounts of each. A low gloss may be achieved, for example, by the use of a combination of dicyandiamide (a slow acting curing agent) and a fast acting agent to set up competitive reactions. A family of substituted ureas that are useful in latent one-pack systems with dicyandiamide is sold under the trademark OMICURE. Curing temperatures in the range of 110-140xc2x0 C. (230-280xc2x0 F.) may be achieved with such one-pack systems. The family includes phenyl dimethyl urea, toluene bis dimethyl urea, methylene bis (phenyl dimethyl) urea, and a cycloaliphatic bisurea designated U-35. The choice of curing agent for a low gloss coating is expanded by the use of acid-functional resins as matting agents in the formulation of the coating powders of this invention. For example, the P-101 imidazole/epoxy resin adduct named as a catalyst hereinabove may be used as a curing agent when such matting agents are used. An accelerated dicyandiamide sold as G 91 by Estron Chemical, Inc. may also be used as a curing agent at low temperatures in the presence of such matting agents to produce low gloss coatings. The OMICURE agents mentioned above may also be used in combination with said matting agents. The amount of low temperature curing agent may also be reduced somewhat in the presence of the acidic matting agents and this, too, has the effect of reducing gloss. As will be shown hereinafter, the smoothness attained by the incorporation of a crystalline epoxy resin, as noted hereinabove, may also be attained in the absence of such a resin when a matting agent is used. Low gloss coatings on wood cured in this manner are among the few that pass the boiling water test of a major furniture maker.
Examples of such matting agents include, without limitation, an acrylic resin having an acid number of 142xc2x15 sold as G 151 by Estron, a polyester having an acid number of about 320 sold as EP-5600 by Ruco Polymer Corp., an acid-functional acrylic resin sold as SCX 880 by S. C. Johnson, and a low molecular weight, tetracarboxyl-functional polyester sold as DT 3357 by Ciba. The amount of matting agent is from about 2 phr to about 20 phr, preferably from about 5 to about 15 phr.
That the choice of epoxy resin is important to the gloss level is shown by the following: A fused and thermally cured powder coating made from an epoxy resin having an equivalent weight of about 400 may have a 60xc2x0 gloss level as low as about 10 whereas a gloss level of about 90 may be achieved when the equivalent weight is about 650 and the curing agent and curing catalyst are changed.
The coating powder may also contain a flow control agent in the range of from about 0.5 to about 2.0 phr. Examples of the flow control agents include the MODAFLOW poly(alkylacrylate) products and others such as the SURFYNOL acetylenic diols (e.g., P200) which contain hydroxyl, carboxyl or other functional groups. The functionalized flow additives also aid intercoat adhesion in the event that touch-up or repair of the powder coating is necessary. The flow additives may be used singly or in combination. Anti-oxidants may also be used at a concentration of from about 0.5 to about 2.0 phr to prevent the discoloration of the coatings even at the relatively low curing temperatures suitable for the purposes of this invention.
Examples of the anti-oxidants that are useful in this invention include sodium hypophosphite, tris-(2,4-di-t-butyl phenyl)phosphite (sold under the trademark IRGAFOS 168), and calcium bis([monoethyl(3,5-di-t-butyl-4-hydroxybenzyl)phosphonate] (sold under the trademark IRGANOX 1425). Mixtures of anti-oxidants may be used. The sodium hypophosphite also acts a buffer against the action of trace amounts of chlorine released by epichhlorohydrin residues in the epoxy resins.
The powder of this invention may be used in coating glass, ceramics, and graphite-filled composites as well as metallic substrates such as steel and aluminum. Much heat is wasted by heating thick or massive metal articles in order to raise the surface temperature to the high levels required by conventional coating powders. The particular utility of the powder of this invention, however, in the coating of heat sensitive substrates such as plastics, paper, cardboard and woods makes it highly appealing as a commercially viable alternative to the liquid coatings that have been almost universally used in the past. The powder coating of plastic parts for the interior and exterior of automobiles exemplifies the utility of this invention. For the purposes of this invention, wood is defined as any lignocellulosic material whether it comes from trees or other plants and whether it be in its natural form, milled, or made into plywood, particle board, or fiberboard of various densities. It is exemplified by lumber, panels, molding, siding, oriented strand board, hardboard, and medium density fiberboard (MDF). The particle board may be standard or treated to enhance its electrical conductivity. Fiberboard having a pattern such as a simulated wood grain printed on its surface, rather than on a paper laminated to that surface, and a powder coating of this invention over said pattern has the appearance of natural wood. MDF is a particularly valuable substrate for said purpose. Wood having a moisture content of from 3 to 10% by weight is suitable for the purposes of this invention. A porous particleboard, pre-coated with a conductive liquid coating composition and cured, may also serve as a substrate for the coating powder of this invention. For example, a smooth 2-3 mil thick powder coating is achieved on a 0.5 to 1 mil thick UV or thermally cured pre-coat.
Pigments, optical brighteners, fillers such as calcium carbonate, texturizing agents such as particulate rubber, bentonite clays, powdered polytetrafluoroethylene (PTFE) with or without polyethylene powders, such as those sold under the trademark LANCOWAX, and other conventional additives may also be present. A particularly desirable textured finish may be obtained by the addition of from about 14 to about 20 phr of the rubber to the coating composition along with calcium carbonate at a rubber to carbonate ratio of from about 0.7:1 to about 1.5:1 by weight. Titanium dioxide, in an amount of from about 5 to about 50 phr or more, is an example of a pigment that may be used. An optical brightener, exemplified by 2,2xe2x80x2-(2,5-thiophenediyl)bis[5-t-butylbenzoxazole], sold under the trademark UVITEX OB, may be present at from about 0.1 to about 0.5 phr.
The coating powder of this invention may be applied by any of the conventional powder coating methods such as immersion of an article in a fluidized bed or in a corona discharge cloud but the application of the powder by triboelectric guns is preferred in particular situations such as when a wooden substrate is profiled. Flat-surfaced as well as grooved panels may be coated by triboelectric guns on a flat line conveyor having electrically conductive bands around the circumference of the conveyor belt. A suitable flat line powder coating apparatus comprises such a conveyor extending through a powder coating booth, wherein a wooden article supported and moved by the conveyor belt is coated triboelectrically by a plurality of guns situated adjacent one another and in one or more tiers. The article bearing the powder is then conveyed through a curing oven having several heating zones, some of which are heated by IR lamps, others by heat convection, and still others by a combination of those two. The coating and curing line speeds may be the same or different depending on the length of the curing oven. The line speed through the powder application booth may be from about 5 to about 150 feet per minute but it is preferably from about 20 to about 100 feet per minute. The line speed through the curing oven, on the other hand, may be from about 5 to about 20 feet per minute, depending on the oven temperature and the particular coating powder used. The curing temperature may range from about 225xc2x0 up to but not including the decomposition temperature of the powder. It is preferred to maintain the cure temperature within the range of from about 225xc2x0 to about 300xc2x0 F. and still more preferred to keep the cure temperature at from about 225 to about 250xc2x0 F. It is preferred that the coating and curing line speeds be adjusted to the oven length so that they are balanced.
Preheating of the panel before the coating step is preferred in some instances, e.g., to help the powder reach its flow temperature in the first zone of the oven and it also minimizes outgassing during cure. The oven may have several heating zones of the IR and convection types and also a combination of the two. The film thickness of the cured coating is at least about 1 mil and it may be as much as about 8 mils or even higher if there is a practical need for such. Film thicknesses of from about 4 to about 6 mils are achieved regularly by the method of this invention.
The gel time of the coating powder of this invention was measured according to ASTM Specification D-3451 (14) in which a small quantity of powder was dropped onto a hot plate at 300xc2x0 F. (149xc2x0 C.) and stroked with a tongue depressor until continuous and readily breakable filaments were formed when the depressor was lifted from the sample. The elapsed time for this to occur was measured in seconds and is the gel time.
The blocking resistance of a coating powder is tested by placing about 1 inch of the powder in a tube, placing a 100 gram load on top of the powder and heating it at 110xc2x0 F. for 24 hours. Upon removal of the contents of the tube, the degree of sintering is measured on a scale of 1 to 10, 1 being completely free-flowing and 10 being non-flowing. Powders having a value of less than 5 are acceptable.
The hot plate melt flow (HPMF) of the powder coating composition of this invention was measured by placing a pellet of powder having a diameter of 12.7 mm and 6 mm thick on a hot plate set at 375xc2x0 F.(190xc2x12xc2x0 C.)at an inclination angle of 35xc2x0. When the pellet melts and runs down the plate, the length of the flow is measured in millimeters. The flow distance is dependent on the initial melt viscosity, the rate of reaction, the temperature at which the test is conducted, and the type and amount of catalyst.
The Hoffman scratch resistance of the coated articles made according to this invention was measured with the Byk-Gardner scratch tester.
The Taber abrasion resistance of the coated articles made according to this invention was measured according to ASTM D-4060 using CS-10 wheels and a 1000 gram load.